In the microelectronics industry as well as in other industries involving construction of microscopic structures (e.g., micromachines, magnetoresistive heads, etc.), there is a continued desire to reduce the size of structural features. In the microelectronics industry, the desire is to reduce the size of microelectronic devices and/or to provide a greater amount of circuitry for a given chip size.
Effective lithographic techniques may lead to a reduction of feature sizes. Lithography impacts the manufacture of microscopic structures not only in terms of directly imaging patterns on the desired substrate, but also in terms of making masks typically used in such imaging. Typical lithographic processes involve formation of a patterned photoresist layer by pattern-wise exposing the radiation-sensitive resist to an imaging radiation. The image is subsequently developed by contacting the exposed resist layer with a material to selectively remove portions of the resist layer to reveal the desired pattern. The pattern is subsequently transferred to an underlying material by etching the underlying material in openings of the patterned resist layer. After the transfer is complete, the remaining resist layer is then removed.
An exemplary lithographic process is depicted in FIGS. 1A-1C. Referring to FIG. 1A, a photoresist layer 30 may be formed above an underlying material layer 10. In some lithographic imagining processes, the photoresist layer 30 may not provide sufficient resistance to subsequent etching steps to enable the effective transfer of the desired pattern to the underlying material layer 10. In such cases, a hard mask layer 20 may therefore be used between the photoresist layer 30 and the underlying material layer 10. The hard mask layer 20 receives the pattern from the patterned resist layer and should be able to withstand the etching processes needed to transfer the pattern to the underlying material. The hard mask may further have anti-reflective properties to improve the resolution of the image patterned in the photoresist by limiting the reflection of the exposing radiation off the underlying layer 30.
Referring to FIG. 1B, the photoresist layer 30 (FIG. 1A) may be developed by, for example, passing exposing radiation 50 through a pattern mask 40 containing transparent portions 40a and opaque portions 40b, resulting in exposed portions 30a and unexposed portions 30b of the photoresist layer 30 (FIG. 1A). In a negative tone development system, the unexposed portions 30b may be removed by a developer while the exposed portions 30a are resistant to the developer.
Prior to developing the photoresist layer 30 (FIG. 1A), the photoresist layer 30 adheres well to the hard mask layer 20, due in part to similar degrees of hydrophilicity. However, after developing the photoresist layer 30 using a negative tone development system, the exposed portions 30a may become hydrophilic, resulting in poor adhesion to the relatively hydrophobic hard mask layer 20. Referring to FIG. 1C, this poor adhesion may lead to image collapse, where the exposed portions 30a do not remain in adhered to the hard mask layer 20. Therefore, a negative tone development system that maintains adhesion between the photoresist and the hard mask after developing the photoresist is desirable.